1. Field of the Invention
The present invention relates to semiconductors, and, more particularly, to low resistance metal contacts for silicon-based devices.
2. Description of the Related Art
The silicon/aluminum metal system has been the metal system of choice for providing metal contacts to silicon-based semiconductor devices for many years. It provides low contact resistance and is relatively stable to back end temperature treatment (e.g., sintering, incorporation in cerdip packages, and the like).
Silicon precipitates have always been associated with silicon-doped aluminum. However, since the contact opening in the past has been comparatively large, contact resistance has not been a problem.
More recently, contact openings are being made increasingly smaller as VLSI (Very Large Scale Integration) technology advances. Consequently, the silicon precipitates are becoming critical to circuit performance.
If the silicon precipitates form in the contact area, contact resistance will increase, due to a reduction in the effective contact area. In the worst case, an open contact is created.
In order to improve step coverage in forming a metal contact layer which must traverse several "steps" created during the formation and processing of various layers, a "hot" metal process is employed, as opposed to the "cold" metal process previously used. In the latter process, the metal is deposited at ambient temperatures. In the more recent hot metal process, on the other hand, the metal is deposited on a substrate maintained at an elevated temperature, typically at about 100.degree. to 300.degree. C.
While the contact morphology is thereby improved, the temperatures associated with the hot metal process increase the likelihood that silicon will precipitate in aluminum.
Bottom layers of metal silicides have been employed in conjunction with overlying aluminum contacts. However, most silicides cannot form a stable ohmic contact with both p.sup.+ and n.sup.+ regions, consequently limiting their use.
Hillock suppression is very important for multi-layer metal contact systems. A top layer of a metal silicide has been found to help suppress the hillock formation. However, excess silicon from the silicide creates an additional potential source for silicon precipitation in the aluminum layer.
Another problem with silicon/aluminum technology is in the area of static random access memories (RAM's). Here, polysilicon resistors are employed, which have resistances on the order of 10.sup.9 ohms per square. However, aluminum contacts to the polysilicon resistors are very sensitive to even low temperature annealing. An anneal at 400.degree. C. for 90 minutes is enough to reduce the resistance to 10.sup.4 -10.sup.7 ohms and thereby cause very high leakage through the polysilicon load resistors, due to the interdiffusion of silicon and aluminum. Such a drastic change in resistance poses yield/product reliability problems and limits the potential layout advantages of using metal as a V.sub.cc line with direct contact to the lightly doped polysilicon resistor load.
It is clear that an improved metal contact configuration is desired, in order to reduce the effects of the formation of silicon precipitates in silicon/aluminum metal contacts.